Change the OP_JournalMode implementation so that it works even if a

substitute sqlite3PagerFilename() that might return NULL is used.

FossilOrigin-Name: 491ff5fb2504173d6905e38b8ea35737338aaa84

src/vdbe.c

@@ -5467,7 +5467,7 @@ case OP_JournalMode: {    /* out2-prerelease */
   ** in temporary storage or if the VFS does not support shared memory 
   */
   if( eNew==PAGER_JOURNALMODE_WAL
-   && (zFilename[0]==0                         /* Temp file */
+   && (sqlite3Strlen30(zFilename)==0           /* Temp file */
        || !sqlite3PagerWalSupported(pPager))   /* No shared-memory support */
   ){
     eNew = eOld;